bug: Array of test helpers in custom watcher gives error

- If a custom watcher returns an array, it will error.
- Flattened the array of paths in run_on_changes that is passed up to super

This is my attempt at fixing [Issue #34](https://github.com/jkutner/guard-jruby-rspec/issues/34) It was a pretty easy fix, and I am relatively sure my test catches what it needs to catch.  Also, I pulled this into my project and it fixed the problem.  

Let me know if I need to take a different approach or if I messed something up.